In general, a lithographic or planographic printing plate includes a lipophilic image area capable of receiving greasy ink and an oil-repellent non-image area which is incapable of receiving ink, but sufficiently hydrophilic to receive water. Lithographic printing is usually carried out by supplying both water and ink to the plate surface so that the image area receives the coloring ink and the non-image area receives water both preferentially whereupon the ink on the image area is transmitted to an ink-receptive sheet, typically paper. For obtaining satisfactory printed matter, it is necessary that there is sufficient differential affinity to oil and water between the image and background non-image areas to ensure that upon supply of water and ink, the image area receives an amount of ink, but the non-image area does not receive the ink at all.
Lithographic printing plates are often prepared by a silver complex diffusion transfer process (DTR). One of their drawbacks is low wear resistance in that the plate is less resistant against mechanical wear so that the hydrophobic area for bearing an ink image may gradually lose its ink receptivity. Another drawback is that the hydrophilic area gradually becomes hydrophobic while the non-image or background area becomes scummed with ink. Further, the hydrophobic image area experiences local losses of even receptivity to greasy ink and thus becomes variant in ink receptivity, resulting in a varying amount of ink being received thereon during printing.
There are known printing plates which utilize a metallic silver pattern formed by DTR process as an ink-receptive site. See U.S. Pat. Nos. 3,220,837 and 3,721,559, Japanese Patent Publication (JP-B) Nos. 16725/1973 and 30562/1973, and Japanese Patent Application Kokai (JP-A) Nos. 4482/1971 and 21603/1978. Some of these printing plates have been used in practice, but not free from the above-mentioned drawbacks.
Like other lithographic printing plates, those plates relying on DTR process are required to enhance plate wear resistance by tailoring the composition of plate material and processing solution and printing conditions. The state of transfer developed silver grains is one of important factors dictating the printing characteristics of plates.
Factors critical for silver grains transfer developed by DTR process to be more resistant against printing wear are conditions of forming transfer silver grains, for example, the diffusion rate, stability and reduction rate of the silver complex, and moreover the size and shape of the thus developed silver grains although other components of the lithographic printing plate have more or less influence.
A number of silver halide complexing agents are known for use in the development step of diffusion transfer process. Examples include (1) thiosulfate salts, (2) thiocyanate salts, (3) aminothiosulfates as disclosed in U.S. Pat. No. 3,169,992, (4) cyclic imide compounds as disclosed in U.S. Pat. No. 2,857,276, (5) 2-mercaptobenzoic acids as disclosed in JP-B 4099/1986, and (6) thiosalicylic acids and cyclic imide compounds having a five or six-membered ring as disclosed in JP-B 4100/1986. Printing plates which are prepared through development with such silver halide complexing agents have the advantages of high sensitivity, high resolution, and high image reproducibility, but are prone to plate wear as compared with other practical printing plates such as presensitized plates. It is thus desired to improve the plate wear of such plating plates.
Among the above-mentioned silver halide complexing agents, thiocyanates are best with respect to the printing wear resistance of transfer developed silver grains and cost. This procedure, however, has several problems including (1) corrosion in that thiocyanates are metal corrosive and thus tend to attack parts of the processing equipment, (2) restricted solution disposal in that thiocyanate solution is classified as cyan-containing solution according to the waste water regulation and must be deliberately treated before discarding to the environment, (3) cumbersomeness in that thiocyanates are deliquescent so that their handling is difficult and their processing solution requires care during preparation, (4) careful management of processing solution in that ink-receptivity largely depends on the amount of thiocyanate added.